She already has. Britney was only consistent for 2 years, and fell off after 3-5 with an occasional comeback hit.
Not with Taylor's local ass. Britney and her brand are global and her impact has yet to be touched. U.S. sales aside, let's see if Taylor is topping Forbes in another decade.
The ridiculous attempts to discredit Britney 
Not to mention going multi-platinum 8 years later in THIS market, is a hard feat. 
